Well, if it's the DB you're worried about all you have to do is make a backup of the data in your DB, redeploy, rake db:migrate, and import your backed up DB file. If you don't you'll purge the data when you migrate.
So if I have a live site with say 2 models that have relationships and I let it run that way for a few weeks and then add 3 more models that have relationships with the first 2 will it break things?
I've just done the migration to my existing site - but no relationships and it's all well. Just a little concerned about down the road.... and figure It's time to make sure I understand so I don't mess myself up.